1. Field of the Invention
The present invention relates to a centrifugal compressing apparatus.
2. Description of the Related Art
As shown in FIG. 7, a centrifugal compressing apparatus 10 has an impeller 1 that is driven to rotate by a motor (not shown), etc., and a casing 2 that houses the impeller 1. The impeller 1 has a hub (rotor) 4 that is formed into a substantially conical shape and blades 3 that are mounted radially onto the hub 4. The casing 2 is formed to a substantially conical-cylindrical shape so as to house the impeller 1 across a predetermined clearance CL. The clearance CL is made substantially fixed in value from a front edge side 5 to a rear edge side 6 of the impeller 1.
Reference symbol H denotes the height of the blade 3, and the height H of the blade 3 is made to decrease gradually from the front edge 5 side to the rear edge 6 side of the impeller 1. The height H of the blade 3 is the amount of protrusion of the blade from the hub surface in a direction orthogonal to the main air flow inside the impeller. In the following description, the value obtained by dividing the amount of change of the blade height with respect to the meridional distance along the hub surface by the meridional distance shall be defined as the blade height changing rate.
At the impeller 1 of the centrifugal compressing apparatus 10, there exists a clearance flow that flows in from the clearance CL between a top edge 7 of the blade 3 and the shroud casing 2. The clearance flow (CLF) refers to a phenomenon, wherein, as shown in FIG. 10, a portion of the air at a pressure surface 3a of the blade 3 of the impeller 1 flows past the clearance CL between the blade 3 and the casing 2 and into the negative pressure surface 3b side of the blade 3.
A modeled flow inside the impeller for an ideal case where the clearance flow CLF does not exist is illustrated in FIG. 8 and FIG. 9. FIG. 8 is a diagram corresponding to a view taken on line A-A of FIG. 7. If, as shown in FIG. 8, it is assumed that the clearance CL does not exist between the blade 3 and the casing 2, the flow velocity distribution (inter-blade flow velocity distribution) of the flow (main flow) flowing in the depth direction orthogonal to the paper surface along the section taken along line B-B of FIG. 8 will, as shown in FIG. 9, be such that the flow velocity decreases gradually from the negative pressure surface 3b side to the pressure surface 3a side of the blade 3.
Meanwhile, a modeled flow for the case where the clearance flow CLF exists is shown in FIG. 10 and FIG. 11. Since the clearance flow CLF flows substantially perpendicular to the main flow direction as shown in FIG. 10, the flow velocity near the negative pressure surface 3b is a mixture of the ideal flow velocity and the substantially zero flow velocity of the clearance flow CLF and thus drops, as shown in FIG. 11, to half the ideal flow velocity shown in FIG. 9. The decrease in flow velocity in the main flow direction resulting from this mixture is a pressure loss.
As shown in FIG. 7, in the impeller 1 of the centrifugal compressing apparatus 10, the height H of the blade 3 decreases from an entrance to an exit in the flow direction. FIG. 12A is a diagram for illustrating a case where the height H of the blade 3 is relatively high, and FIG. 12B is a diagram for illustrating a case where the height H of the blade 3 is relatively low. Since the clearance CL between the blade 3 and the casing 2 is substantially fixed from the front edge 5 side to the rear edge 6 side of the blade 3 as mentioned above, when the height H of the blade 3 decreases, the ratio of the width Δb of the clearance CL to the height H of the blade 3 (Δb/H) becomes relatively large, and thus the ratio of the area occupied by the clearance flow CLF to the area occupied by the main flow becomes large as shown in FIG. 12A and FIG. 12B and thus the pressure loss increases. The pressure loss due to the clearance flow CLF is greater the lower the height H of the blade 3, and is greater at the rear edge 6 side than at the front edge 5 side of the blade 3.
Japanese Published Unexamined Patent Application No. 2000-64998 discloses a centrifugal compressing apparatus, wherein an abradable layer that is abraded by an impeller is provided on an inner surface of a casing that houses the impeller, and with this centrifugal compressing apparatus, when the length from a front edge to a rear edge of the impeller along the inner surface of the casing is M and a length from the front edge of the impeller to an arbitrary position is m, the abradable layer is disposed in the range of M-m, with 0.2≦m/M≦0.4 being satisfied.